Open with "mpv Media Player"
Open with "Mousepad"
Open with "Enqueue in Audacious"
Open with "Geany"

How can I set up my system that Open with "mpv Media Player" is also listed on the main right mouseclick (or context menu key) menu, so that I have two associations there, like a text file is associated with both geany and mousepad?

The Trick with adding [playlist] doesn't work. Thats the default first line for .pls playlists but I use .m3u for video playlists. I wrote a script that only adds video files from a folder into a playlist file named just like the folder. Doing so with .m3u is the easiest cause it not needs converting many Charakters like whitespace 》%20 or such.

I tried the other approach but it fails for xfce V2018.01.14

I have the other programs but audacious in a submenu, even when I tell Thunar that mpv should be the default... It sure loads the m3u into mpv... but on the main context menu there still is only audacious listed being associated with m3u...

This could well be -- because:
The latest version i have is 003-xfce-x86_64-2018-01-02.xzm.
It doesn't even boot into Desktop, i'm stuck at the login screen.
with ctrl+alt+F* i can login but not start x:

I was partly wrong with what I said about m3u playlist.
You have to exit the folder in Thunar and reenter the folder. I thought "reload" would do the trick but no...
Anyhow then you see mpv as default app for m3u in the main context menu window... but audacious is removed. I want both audacious and mpv for m3u files to appear on the main menu.

V2018.01.14 and V2017. 12.16 act the very same. I have several programs to choose for .pls but only ever one for .m3u.

(defaults)
right-click a video file (webM/mp4) = open with mpv
right-click a audio file (mp3/wav) = open with audacious

Let's create a *.m3u playlist which contents only video files.

Because i am lazy i use the playlist window of audacious.
It doesn't matter if audacious is able to actually play these files (don't try),
one can still put files in this window and export the playlist as:
m3u , pls , xml shareable playlist (xspf) , asxv3 playlist or audpl (audacious playlist)
Let's choose m3u.

Now right-click that file = only audacious and leafpad.

right-click again > open with other application, select mpv media player,click open =
videos are playing.-- I now have the mpv context menu entry like shown above when
right-clicking a m3u file, no matter if they contain audio or video files.

The same work the other way round:
If i have a *.pls file mpv is the default > right-click,other app,audacious >
both are in context menu.

This context menu thing is set through mime types, and it's a pain in the ass to correctly set. The idea is to put the mime types (essentially file extensions) you want inside /usr/share/applications/[app].desktop at the mime type section. To set the default program for each mime type, you should look at /usr/local/share/applications/default.list -- the first application on the list of each mime type corresponds to the default application that will open when you double click a given file extension.

This context menu thing is set through mime types, and it's a pain in the ass to correctly set. The idea is to put the mime types (essentially file extensions) you want inside /usr/share/applications/[app].desktop at the mime type section. To set the default program for each mime type, you should look at /usr/local/share/applications/default.list -- the first application on the list of each mime type corresponds to the default application that will open when you double click a given file extension.

If my explanation isn't clear, please let me know.

Thanks for the heads up. .. I try to look into this info.

I already have set up some mime types when adding viewnior as my default image viewer instead of gpicview. .. and I also load the mtpaint module by default.

Maybe all these changes are somehow messing up the handling of m3u Files?
Not that image files and m3u files are related. .. but in this issue all are still just mime types...